The exhaust gases of internal combustion engines contain pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx) that foul the air. Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants have been set by various governments and must be met by older as well as new vehicles. In order to meet such standards, catalytic converters containing a three way catalyst (TWC) may be located in the exhaust gas line of internal combustion engines. The use of exhaust gas catalysts have contributed to a significant improvement in air quality. The TWC is the most commonly used catalyst and it provides the three functions of oxidation of CO, oxidation of unburned hydrocarbons (HC's) and reduction of NOx to N2. TWCs typically utilize one or more platinum group metals (PGM) to simultaneously oxidize CO and HC and reduce NOx compounds. The most common catalytic components of a TWC are platinum (Pt), rhodium (Rh) and palladium (Pd).
The platinum group metals (PGM) in the TWC catalysts (e.g., platinum, palladium, rhodium, ruthenium and iridium) are typically dispersed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating, or on an oxygen storage component (OSC), or their mixtures. The support is carried on a suitable carrier or substrate such as a monolithic substrate comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The TWC catalyst substrate may also be a wire mesh, typically a metal wire mesh, which is particularly useful in small engines.
Refractory metal oxides such as alumina, rare-earth metal oxides, zirconia, titania, and their combinations, and other materials are commonly used as supports for the catalytic components of a catalyst article and as oxygen storage materials (OSC). Currently, almost all of the alumina catalyst supports and OSC are in the form of solid powder particles with a particle size ranging from about 5-100 microns or are large extrudates above 100 microns in size. The alumina support materials typically exhibit a BET surface area in excess of 60 square meters per gram (“m2/g”), often up to about 200 m2/g or higher.
In an internal combustion engine it is also desirable for these catalyst support materials to have high meso- and macro-porosity to enhance gas phase diffusion, which makes the catalysts more effective to achieve high nitrogen oxide (NOx) and hydrocarbon (HC) conversion at high space-velocity. In this regard, porous microspheres, including hollow microspheres, have been used as catalyst supports for the purpose of improving the porosity of the catalytic washcoat. Various preparation methods for such microspheres are reported in the literature. However, in general, hollow alumina microspheres that are formed at lower temperatures are thin-walled egg-shell structures that are too fragile to resist mechanical milling during catalyst preparation and hydrothermal aging in the engine. Thick-walled hollow alumina spheres are more robust against mechanical and thermal aging, and are available commercially (e.g., as insulation material), but these materials either have a large particle size or have been sintered at too high a temperature for catalyst applications. Hollow alumina microspheres made using ion-extraction of boehmite sols followed by firing at 1200° C. have been shown to have thick walls; however, these microspheres are in the dense alpha crystalline phase.
There remains a need for hollow porous microspheres suitable for use as catalyst supports which can be made by simple manufacturing methods, and which have thick walls with small spherical diameters. The availability of such microspheres also leads to a significant reduction in raw material usage (e.g., precious metal, alumina, and OSC) and therefore a substantial reduction in cost because of elimination of the dead space in the center of the conventional solid particle.